An air CO2 capture system based on the passive carbonation of large Ca(OH)2 structures

Direct Air Capture (DAC) requires contacting a vast flow of air with a functional surface, which must be accommodated in a large and costly CO2 capture device (i.e. at least 0.2–0.4 m3 of the reactor volume per tCO2 per year). We propose in this paper a low-cost alternative that involves contactor volumes that are one or two orders larger, but require only the passive CO2 carbonation of purpose-built porous structures of Ca(OH)2. Such low-cost materials can be manufactured from natural limestone and/or from recycled carbonated structures by using oxy-calcination technologies, and then simply stacked in such a way as to leave gaps for air to pass through. On the basis of an analysis of the rate controlling factors of the carbonation reaction, we employed as the structural element sintered Ca(OH)2 plates with an area of 2 × 2 m2 and 3 cm thick with a porosity of 0.5, which can be fully carbonated in about 6 months. The cost of CO2 captured from air is estimated to be between 140 and 340 $ per tCO2 depending on assumed cost values for fuel, land use, structural materials manufacture & transport, and process and project contingencies.